(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of generating alignment marks only on the oxide layers in the fabrication of integrated circuits.
(2) Description of the Prior Art
In the fabrication of integrated circuit devices, multiple layers of conductors and insulators are deposited and patterned to construct the integrated circuit. It is critical to align each subsequent layer with the previous layer with precision. This is typically accomplished by using alignment marks. A wafer stepper tool uses the alignment marks on a wafer as a reference point for adjusting a reticle over the wafer. The reticle contains the pattern to be generated within the layer. The reticle must be precisely aligned to the previous layer. A wafer stepper uses one of at least three methods to detect the alignment marks; these are light interference, bright field contrast, or dark field polarization effect.
Some methods generate alignment marks within the scribe line area. Scribe lines define a cutting portion where devices formed on a wafer are cut apart after manufacture. Some alignment mark schemes require fresh alignment marks to be generated at every layer, using the previous layer's marks to assure alignment. Alignment marks occupy more and more space as the number of interconnect layers increases and scribe line area becomes scarce. Some alignment schemes transfer the alignment marks from layer to layer. However, this cannot be done when planarization is performed, such as chemical mechanical polishing (CMP). In this case, the alignment marks must be recovered after CMP. These recovery or repair steps lengthen the process cycle and increase the production costs.
U.S. Pat. No. 5,503,962 to Caldwell discloses an alignment mark and CMP process in which alignment marks are formed in oxide layers using the same process as for contact and via formation. However, there is no mention of how to generate a quality alignment mark or of how to use the alignment mark beyond the next layer. U.S. Pat. No. 5,663,099 to Okabe et al teaches a method of forming an alignment mark. U.S. Pat. No. 5,401,691 to Caldwell teaches a method of recovering alignment marks after CMP. U.S. Pat. No. 5,496,777 to Moriyama teaches forming an alignment mark for each layer in a widened portion of a scribe line. U.S. Pat. No. 5,648,854 to McCoy et al discloses an alignment system to detect the wafer edge and global alignment marks.